Compressive Strength of Temporary Crowns Made from Default design in Two Types of Software

Statement of the Problem: Various default values in each software can eventually lead to different crown thicknesses and affect their compressive strength. Purpose: This study aimed to compare the compressive strength of temporary crowns made with the milling machine, designed in Exocad and 3Shape Dental System software. Materials and Method: In this in vitro study, 90 temporary crowns were made and evaluated based on each software settings. For this purpose, a sound premolar was first scanned as a pre-operative model by 3Shape laboratory scanner. The standard tooth preparation and scanning were performed, and then the temporary crown files (designed by each software) were transferred to the Imesicore 350i milling machine. A total of 90 temporary crowns (45 based on the file of each software) were made using poly methyl methacrylate (PMMA) vita cad-temp block. The value of compressive force displayed on the monitor was recorded at the first crack and the ultimate failure of the crown. Results: The first crack and the ultimate strength of crowns designed with the Exocad software was 903.5±96N and 1490±139.3N and for crowns designed with 3Shape Dental System software was 1060.4±160.2N and 1691.1±73.9N, respectively. The amount of compressive strength of temporary crowns made with 3Shape Dental System was significantly higher than those made with Exocad software and this difference was statistically significant(p= 0.000). Conclusion: The compressive strength of temporary dental crowns made by both softwares is in a clinically acceptable range, but considering that the average compressive strength in the 3Shape Dental System group was slightly higher than of the other group, it is preferable to design and fabricate with 3Shape Dental System software to increase the compressive strength of these crowns.


Introduction
The loss of one or more teeth in patients, leads to functional problems such as poor chewing and speech, as well as inappropriate esthetic, self-image destruction, and loss of emotional balance, which subsequently would affect their quality of life [1]. Increasing patients' demand for maintaining the beauty and function of their dental system has led to the increasing use of dental prosthesis [2]. Fixed prosthodontics treatments can range from the restoration of a single tooth to full occlusion reconstruction. Replacing lost teeth with fixed prostheses can increase the comfort and ability to chew, ensure the health of dental arches, and in many cases increase the patient's self-confidence [3]. In addition, using fixed prostheses and providing an optimal occlusion can improve the orthopedic stability of the tem-poromandibular joint [3]. Temporary crowns are used as an essential component in treatments, which are based on fixed prosthodontics and implants. A temporary crown has many advantages such as pulp protection, periodontal health, maintaining the position of the tooth and preventing its displacement or the opposite tooth [4]. A temporary crown generally protects the prepared tooth until the definitive restoration is delivered [4].
Various methods for prosthetic reconstruction of edentulous spaces or repair and reinforcement of severely damaged teeth have been introduced [1,5]. In general, these methods can be divided into two categories as traditional and digital [5]. The traditional method is based on the use of conventional impression materials and laboratory procedures such as casting and wax pattern making and so on [6], which in addition to having more errors, it has a long process of treatment [7]. The digital method has several benefits such as increasing the adaptation of the definitive prosthesis and patient comfort in the treatment process, reducing prosthesis preparation time and at the same time the risk of human and laboratory errors. It also reduces the cost of treatment [8]. Digital dentistry consists of three phases including digitizing the teeth and oral environment by intraoral or laboratory scanners, design of restoration or oral appliance in specialized computerized equipment (CAD), and conversion of digital design into physical model (CAM) [9]. Digital dentistry, in turn has a variety of methods for restoring lost structures using various softwares. The Exocad and 3Shape Dental System are some of the most widely used software in this field.
This software offers different components in the design, which can be by default or be changed in any software environment [10].
The software defaults for designing crown in Exocad and 3Shape Dental System software can lead to different spaces between the inner surface of the crown and the outer surface of the prepared tooth. This can subsequently lead to different crown thicknesses in a standardized anatomy and affect its compressive strength [11]. Given the lack of sufficient studies in this regard, we decided to study and compare the compressive strength of temporary crowns made with milling machine resulting from the design in two Exocad and 3Shape Dental System softwares, and present the results in this article. The hypothesis of this study was that temporary crowns made from the default design in Exocad software exhibit similar strength as 3Shape Dental System software.

Results
Examination of the compressive strength of the studied temporary crowns showed that in the temporary crowns made with the milling machine resulting from design in the Exocad software, the average force required to create the first crack was 903.5±96 N ( Figure 1) and for the ultimate failure, it was equal to 1490.4±139.3N ( Figure   Figure 1: Distribution of required force for the first crack in temporary crowns based on Exocad software 2). However, for the samples made from design in the 3 Shape Dental System software, the average force required to create the first crack was equal to 1060.4±160.2 N ( Figure 3) and for the ultimate failure, it was equal to 1691.1±73.9 N (Figure 4).
The independent t-test indicated a significant difference (p= 0.000) between the amount of force required to create the first crack and the ultimate failure in tempora-    (Table 1).

Discussion
The null hypothesis was rejected. It was shown in the present study that the amount of compressive strength of temporary crowns made by 3Shape Dental System software was higher than Exocad software. The 3Shape Dental System is a complete and integrated CAD/CAM for designing prosthetic restorations; this software automatically scans images and completely reconstructs 3D models [13]. In addition, the 3Shape Dental System has the ability to prepare 3D models based on the type of materials required by technicians, dentists, and patients [13]. In general, it has been optimized for frameworks, crowns, and bridges fabrication [13]. The main advantages of this software are doing projects at high speed, the ability to view virtual 3D models without wasting materials and no need for special preparation for the design of crowns [13].
Each software has its own default values for marginal gap, cement space, and crown thickness and so on.
These defaults are stored in the software memory for different employed materials and the type of crown. These assumptions are defined for standard conditions to achieve the best clinical results. It is worth noting that in clinical situations, some of these defaults can be changed and customized depending on the number of abutments, preparation convergence and possible undercuts [10].
For example, in 3shape software, a space of a few microns is defined in the one-millimeter strip of the margin area, which causes acceptable compliance of restoration according to the software designers. Moreover, in the same software to avoid interference of the milling machine's burr, in point angles that the diameter of which is thinner than the milling burr diameter, by default, the drill compensation option is used, which ultimately reduces the wall thickness in that place, to compensate the drill in that small area; and according to the software designer, this extra amount of cutting has a significant effect on matching the inner surface of the crown and teeth in that area [15]. The cement gap and extra cement gap default standard for manufacturing PMMA provisional crowns in 3shape dental system software is 0.04mm and 0.85mm and in Exocad software is 0.04mm and 0.08mm, respectively [15]. All temporary crowns that have been tested had the minimum thickness of 0.8mm, which is necessary for provisionals.
The study of Reeponmaha et al. [14], which com-

Conclusion
In the present study, the results showed that the compressive strengths of temporary dental crowns are clinically acceptable by both softwares. However, because the average compressive strength in a standardized anatomy in the 3Shape Dental System group was slightly higher than the other group, it is recommended to design and manufacture temporary dental crowns by using this software.